1. Field of the Invention
A membrane process and apparatus for economically converting Na.sub.2 SO.sub.4 to H.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 on a large scale. The system is particularly useful where the Na.sub.2 SO.sub.4 that is thus converted is extracted from ores. More particularly, the present invention is concerned with an apparatus and process that for said conversion utilizes a stack of many cells disposed between a cathode and an anode, the cells constituting a set of serially arranged pairs of acid and base zones of which each pair includes a pair of cation exchange membranes and an intermediate bi-polar membrane having a cation permselective face and an anion permselective face, as well as electrolytes which are different on opposite sides of the bi-polar membrane. Adjacent cells share a common cation exchange membrane.
2. Description of the Prior Art
The use of membrane systems for purification and conversion of sundry chemicals is well known. Anionic, cationic and bi-polar membranes are commercially manufactured by large companies. By way of example, attention is directed to U.S. Pat. Nos. 4,219,396 and 4,238,305 of Allied Chemical Corporation which disclose two versions of electrodialytic processes. The earlier of these relates to the electrodialysis of aqueous NaCl to produce high strength, high purity Na.sub.2 CO.sub.3 and either high purity HCl or CaCl.sub.2. In this process, a unit cell is employed having anion permselective, cation permselective and bi-polar membranes wherein the solutions on both sides of the anion permselective membranes are acidic. In U.S. Pat. No. 4,238,305 aqueous solutions of Na.sub.2 CO.sub.3, NaHCO.sub.3, trona (NA.sub.2 CO.sub.3 . NaHCO.sub.3 . 2H.sub.2 O) or mixtures thereof are converted into NaOH and CO.sub.2. In the system of the '305 patent, an electrodialytic cell is employed having an anode compartment and a cathode compartment separated by base and acid zones between which a bi-polar membrane is located. Mention is also made of U.S. Pat. Nos. 3,220,941 and 3,438,879 which disclose the use of permselective membranes for the electrolysis of NaCl solutions to produce hydrogen gas, chlorine gas, NaOH, and Na.sub.2 CO.sub.3. Other electrolytic processes are disclosed in U.S. Pat. Nos. 3,383,080; 3,787,304; 3,963,592 and 4,082,835. None of these is concerned with electrodialytic cells for producing H.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 from Na.sub.2 SO.sub.4.
Na.sub.2 SO.sub.4 is the commercially principal constituent of certain ores obtained from mines located in various parts of the world. For example, Na.sub.2 SO.sub.4 ores are mined to a limited extent in Northern Chile, the ores usually being shipped, sometimes with refining but no conversion into useful chemicals, to other countries. Specifically, Na.sub.2 SO.sub.4 occurs in great amounts in an anhydrous condition as thenardite at Tarapaca, Chile, and also is obtained from existing resources in the nitrate deposits of Pedro de Valdivia and Maria Elena, as well as in the Atacama salt flat, all in Chile's Great Northern Region. Other Na.sub.2 SO.sub.4 ores of low and high grade are found in different parts of the world, examples being Canada, California and Arizona.
It would represent a great saving in manpower, energy, handling and transportation if the extraction of Na.sub.2 SO.sub.4 from ores principally containing the same, and its conversion into useful chemicals such as H.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3 and, optionally, S and NaOH, could be carried out in a simple, labor-efficient and energy-efficient manner on a large scale with relatively inexpensive equipment which does not require a high degree of expertise or engineering.
It is known that Na.sub.2 SO.sub.4 can be reduced with the use of coal or natural gas to Na.sub.2 S in accordance with the following equations: EQU Na.sub.2 SO.sub.4 (s*)+2C(s).fwdarw.Na.sub.2 S(s)+2CO.sub.2 (g**) (1) EQU Na.sub.2 SO.sub.4 (s)+CH.sub.4 (g).fwdarw.Na.sub.2 S(s)+CO.sub.2 (g)+2H.sub.2 O (2) FNT *(s) indicates a substance in the form of a solid. FNT **(g) indicates a substance in the form of a gas. Where coal is used--equation (1)--the Na.sub.2 S taken from the furnace is leached with water and the residue discarded to waste.
One of the difficulties attendant on electrodialysis is that although a permselective membrane is supposed to pass a given type of ion in one direction, as a matter of practice the permselective membrane also permits opposite types of ions to pass through it in the other direction, although to a lesser extent than the direction of the ions for which the permselective membrane was designed and intended. For example, a cation permselective membrane might be principally designed so that a given cell containing a solution of NaOH will pass sodium ions preferentially through such membrane toward the cell cathode. However, such membrane will also pass a lower proportion of hydroxide ions toward the cell anode. The back flow results in a considerable drop in efficiency of the electrodialytic cell.
For the kind of conversion system to which the present invention relates, that is to say, the system conversion of Na.sub.2 SO.sub.4 to H.sub.2 SO.sub.4 and Na.sub.2 CO.sub.3, the difficulty just mentioned with respect to the cell would prevail as a result of the use of a bi-polar membrane except for the inclusion of certain new concepts that are incorporated in the manner set forth hereinafter.